1. Field of the Invention
This invention relates to radio navigation receiver apparatus for use in a simulated Doppler radio navigation system.
2. Description of the Prior Art
Briefly, a (simulated) Doppler radio navigation system (Doppler Microwave Landing System) is one in which there is a ground beacon transmission format involving (a) commutation of a first radio frequency discretely and, in turn, to a linear array of antenna elements, the array being typically of 60.lambda. electrical length, so as to simulate constant velocity unidirectional or bidirectional scanning of the array, whereby a main bearing signal is transmitted which, as "seen" by a remote receiver of the system, undergoes a Doppler frequency shift proportional to the sine of the angle subtended by the receiver normal to the axis of the array; and (b) simultaneous transmission of a reference signal of a second radio frequency (offset from the first), whereby the remote receiver is able to derive a Doppler beat waveform having a frequency indicative of a navigational angle. A basic system of the type is described in U.S. Pat. Nos. 3,626,419 and 3,670,337.
A feature of the above-described transmission format is that, in an elevation guidance system wherein the commutated array is vertical, the direct path signal as received by a remote radio receiver of the system may, in principle, be distinguished from a multipath signal; i.e., a signal which has been reflected from the ground, for example, because the Doppler frequency shift imparted to the direct path signal is of the opposite sense, vis-a-vis that imparted to the ground reflected signal. Accordingly, the receiver fundamentally needs to include a wideband Doppler information filter having a passband covering the range of anticipated Doppler beat frequencies of the direct path signal, with the multipath signals lying in the stop band outside the passband of the filter. Typical beat frequencies for a Doppler elevation system corresponding to direct path signals are 14.88 KHz at 0.degree. elevation down to 8.22 KHz at 10.degree. elevation. The ground multipath signal may be typically between 14.88 KHz (0.degree.) and 21.54 KHz (-10.degree.). The (desired) information filter therefore would have a passband of 8.2 to 14.8 KHz.
However, the side-lobes of the reflected signals spread throughout the passband of the information filter, as will be seen in FIG. 1 of the accompanying drawings, and produce significant error in bearing indication for flight paths over the whole sector when a zero crossing detector and counter are used to register the total number of counts occurring over a complete digitization period. (This is described in British Pat. No. 1,234,541.)
A further error occurs, namely that due to frequency pulling the beat signal towards the center frequency of the filter. This characteristic is well known and is referred to as filter truncation. These two effects are essentially independent of one another but combine linearly.
In order to reduce the aforementioned two errors, the apparatus described in U.S. Pat. No. 3,946,386 provides a radio navigation receiver for use in a Doppler radio navigation system having a transmission format with digitized scanning as hereinbefore defined, said receiver including first means for deriving a Doppler beat information signal from the received signals and including a wideband Doppler information signal filter having a bassband covering the range of anticipated Doppler beat frequencies of the direct path signal, second means for determining the time at which each scan is commenced, third means synchronized by said second means for determining the frequency of said Doppler signal during each said scan, and fourth means under control of said second means for inhibiting operation of said third means at each said scan commencement for at least part of the transient response time of the filter.
However, such a receiver works best when operating at elevations above the horizontal; i.e., at heights above the phase center of the transmitting antenna aperture which, in the case of a microwave landing system operating at C-band, may be not less than approximately 17-18 feet. Current requirements dictate that accurate guidance (i.e., with less than 2 ft. error) be achieved over a 2200 ft. touchdown range with a receiving antenna height on the aircraft as low as 8 ft. above the runway, resulting in a so-called "look down" elevation angle. FIGS. 2a and 2b of the accompanying drawings illustrate, respectively, plan and side views of the placement of the elevation ("flare") antenna FA relative to a runway. The relevant requirements for a typical placement are:
______________________________________ Slant range from antenna 2750 ft. 700 ft. Multipath separation angle 0.33.degree. 1.31.degree. Angular accuracy required 0.042.degree. 0.164.degree. Look down angle ##STR1## ##STR2## ______________________________________
where h.phi. = ht of antenna phase center.
Consider, for example, the requirements for a receiver sector filter suitable for a 144.lambda. system. A 144.lambda. antenna has an equivalent beamwidth of 57.3/144 = 0.4 degrees. Additionally, in the case of a C-band system where .lambda. = 0.2 ft., the phase center of the antenna will be 17.4 ft. above the ground (allowing 3 ft. clearance under the antenna for snow). The angular requirements in terms of antenna beamwidth (bw) for such an antenna are:
______________________________________ Slant range from antenna 2750 ft. 700 ft Multipath separation angle 0.83 bw 3.275 bw Required accuracy 0.105 bw 0.41 bw Look down angle 0.490 bw 1.92 bw ______________________________________
In order that the worst case error at 0.85 bw separation is less or equal to 0.105 bw (required at 2750 ft. range) the ground reflection must be effectively reduced to -12dB with respect to the direct signal. This results from theory relating to accuracy of narrow band tracking devices used in the Doppler system for finally measuring the signal position.
The ground reflection coefficient to be encountered at low angles will be as relatively strong (compared to the direct signal) perhaps as much as -1 dB. Thus, the net attenuation to be obtained is 11 dB. The use of a gated sector filter can provide such an attenuation, but the attenuation value obtained must be considered in conjunction with the "aperture shortening" effect of the gate. In such a case, the gating-in duration will be about 85% of the total scan time, and this increases errors by about 1.5 dB. So the filter cut-off must achieve 12.5 dB attenuation within 0.85 bw frequency shift.